This invention relates to resistors, and more particularly, to film resistors capable of operating at high voltage, as well as compositions for making same.
Pyrochlore is a mineral of varying composition generally expressed as (Na,Ca).sub.2 (Nb,Ti).sub.2 (O,F).sub.7, but which approaches the simpler formulation NaCaNb.sub.2 O.sub.6 F. The structure of the mineral, established by characteristic X-ray reflections, has a cubic unit cell with dimensions of about 10.4 Angstroms and contains eight formula units of approximate composition A.sub.2 B.sub.2 X.sub.6-7. The term pyrochlore is used interchangeably herein with the term pyrochlore-related oxide to mean oxides of the pyrochlore structure with the approximate formula A.sub.2 B.sub.2 O.sub.6-7. Compounds of the pyrochlore-related (cubic) crystal structure are known to be useful as resistors. See, for example, Schubert U.S. Pat. No. 3,560,410, issued Feb. 2, 1971; Hoffman U.S. Pat. No. 3,553,109, issued Jan. 5, 1971; Bouchard U.S. Pat. No. 3,583,931, issued June 8, 1971; Popowich U.S. Pat. No. 3,630,969, issued Dec. 28, 1971; Bouchard U.S. Pat. No. 3,681,262, issued Aug. 1, 1972; and Bouchard U.S. Pat. No. 3,775,347, issued Nov. 27, 1973; each of which is incorporated by reference herein.
Such pyrochlore-based resistors have often been found to have deficiencies when compounded to achieve high resistivities. The high voltage handling capability of film resistors is important, since in certain demanding high voltage uses a resistor may operate at a voltage stress in the range 1000-3000 volts/inch (40-120 volts/mm), and may be exposed to brief (less than one second duration) voltage surges up to 30 kilovolts/inch. As a result of such a voltage surge, most resistors exhibit a permanent change in resistance of up to 50% of their pre-surge lower operating voltage resistance. Resistors are needed which can undergo high voltage surges without undergoing such large changes in resistivity.
The resistivity of presently available high resistivity resistors is normally quite dependent on the concentration of the conductive phase. Therefore, resistor compositions less dependent upon variations in concentration of the conductive phase are needed.
Thus, improved resistor compositions and resistors are needed where high resistivity (1 to 10 megohm per square) are desired, for example, in high voltage applications such as voltage divider networks, focus potentiometers, and other electrical networks.